Working from an office suite behind a Burger King in southern Virginia, operatives used a web of shadowy cigarette sales to funnel tens of millions of dollars into a secret bank account. They weren’t known smugglers, but rather agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The operation, not authorized under Justice Department rules, gave agents an off-the-books way to finance undercover investigations and pay informants without the usual cumbersome paperwork and close oversight, according to court records and people close to the operation.